A variety of applications, including many forms of wireless communications, employ radio-frequency (RF) transmissions. For example, an RF carrier wave may be modulated with a baseband signal containing information to be communicated. The modulated RF signal may then be amplified and transmitted. A certain degree of care must be taken in the amplification of the RF signal to ensure that it is not distorted. If the RF transmission is distorted, a receiver may not be able to properly demodulate the RF signal and recover the original baseband signal.
To avoid distortion, the amplification process must remain relatively linear. One way of accomplishing this is to use a highly backed-off amplifier (i.e., an amplifier operated well below its maximum power capacity). By backing off the amplifier, it may be operated in a relatively linear fashion. However, amplifiers typically operate very inefficiently when backed off far from their maximum capacity.
Another way to avoid distortion is to use feedback to correct for nonlinearities in the amplified RF signal. The principal of feedback, specifically negative feedback, has been known for decades. High-fidelity audio amplifiers typically use large amounts of negative feedback to reduce audible distortion to very low levels. In principal, direct negative feedback (in which the output is connected back to the input via a passive feedback network) can also be applied to RF amplifiers. However, very high system gain is required to obtain significant linearization. It is relatively easy to obtain high system gain at audio frequencies, but it is not so easy at radio frequencies. As a result, the use of direct negative feedback has been limited in RF power amplifiers, and has provided only marginal results.
As an alternative to direct negative feedback of an RF signal, the RF signal may be converted to baseband (i.e., a low-frequency signal, similar to audio frequencies) at which high gain may be obtained. The highly amplified baseband signal may then be converted back to RF and applied to the input of an RF amplifier. This process requires demodulating the signal from RF to baseband and then modulating the amplified baseband signal back to RF.
In a Cartesian feedback system, the demodulation process produces “in-phase” and “quadrature,” or “IQ” signals. Cartesian feedback system are sensitive to operating conditions such as temperature and require IQ demodulators and modulators with very tightly controlled phase characteristics. To control the phase characteristics properly, Cartesian feedback requires a complex auxiliary control system that is difficult to implement and adjust. In addition, systems employing Cartesian feedback typically are expensive and complex. As a result, Cartesian feedback is not desirable for certain types of RF applications, such as two-way radio communication.
Accordingly, there is a need for efficient systems for linear amplification of RF signals. In addition, there is a need for relatively simple, inexpensive linear RF amplification systems.